Rooms don't form a cohesive set of channels that one can join, rather the group together a bunch of channels. If I, for example, setup a #rules channel in a community room, then I cannot ensure that users are reading this channel, which contains vital information about the operation of the community. Same for announcement channels for events. Instead of people joining a server community, they join a bunch of bubbles formed by the people using only subsets of the channels, without being able to direct the overall community. This is poor UX for both the user and the community operators.